I think, there are more ways for skill edge to show in MTTs. You have deeper stacks in the beginning, and even later on average stack size will be larger on average, unless its a turbo. There is also the fatique factor especially in large MTTs, where you might have played 8 hours before reaching the final table. The player best able to handle this has a great edge.
SnGs are more mechanical and close to solved, so the cap on ROI is almost certainly lower. But as others have said, you reach the long term faster. SnGs are also a good way to practice for MTTs, because you get to the bubble and short handed play much more often.
If you play 1.000 player MTTs, you might have played 100, before you get to a final table. And since the money at stake is now many times your original buyin, its nice to have some experience with this situation from smaller tournaments like SnGs.